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A power transmission chain transmits power from a driving sprocket to one or more driven sprockets by forming an endless loop that partially wraps and engages the teeth of sprockets. Rotation of the driving sprocket moves the chain thereby transmitting power through the chain to rotate the driven sprockets engaged by the chain. Power transmission chains are widely used in the automotive industry. In an engine timing application, a chain transmits power from at least one driving sprocket positioned on a crankshaft to at least one driven sprocket positioned on a camshaft. Other automotive applications include, transmitting power from a torque converter to a transmission and transmitting power in the transfer case of a four wheel drive vehicle. Power transmission chains are also widely used in industrial applications.
One type of power transmission chain is known as a xe2x80x9csilent chain.xe2x80x9d A typical silent chain comprises links that are adjacent to each other along the chain. Each link conventionally forms two teeth that are adjacent to each other along a chain direction of the link, that extend in a front direction that is perpendicular to the chain direction, and that are formed to engage the teeth of a sprocket. Links of a silent chain also conventionally form two apertures, one near each end of the link along the chain direction, that extend through the link in a lateral direction that is perpendicular to the chain direction and perpendicular to the front direction of the link. The links of a silent chain are typically formed by a row of substantially identical flat links, each forming teeth and apertures as described, that are positioned laterally adjacent to each other to collectively form a link of the chain.
A silent chain is formed by positioning rows of flat links adjacent to each other and partially overlapping along the chain directions of adjacent chain links so that apertures at adjacent ends of links are aligned. Pivots, such as pins, extend through the aligned apertures to pivotally join adjacent links. Rows of links are joined in this manner to form an endless loop in which the front direction of the links, the direction that the teeth extend from the chain links, is directed toward the region within the endless loop formed by the chain. Links having teeth extending into the region within the loop are referred to as inverted tooth links. The inner surface of the chain, along which the teeth extend, is referred to as the front side of the chain. The surface of the chain opposite the front side facing outwardly from the region within the endless loop is the back side of the chain.
The inverted tooth links (sometimes referred to as driving links) transfer power between the chain and sprocket along the chain direction. Each tooth of a link defines an inside flank that faces generally along the chain direction toward the adjacent tooth of the link, and an outside flank that faces away from the inside flank of the tooth. The inside flanks of the teeth of an inverted tooth link meet at a crotch between the teeth. The teeth of a link may contact sprocket teeth along their inside flanks or along their outside flanks, or along both flanks. The contact between a link tooth flank and a sprocket tooth may transfer power or may be an incidental contact. An inverted tooth link conventionally may contact a sprocket on a side of a sprocket tooth or at a root between adjacent sprocket teeth.
A row of inverted tooth links is positioned on a sprocket by contact with three sprocket teeth, one at each end of the row along the chain direction, and one between the teeth of the links of that row. A row of links is positioned at each location by contact with the sprocket by the teeth of the links comprising the row contacting the sprocket, by the adjacent teeth of the links of the adjacent rows, or by both. Contact with the sprocket at three locations along the chain direction of the row links limits motion of the row of links on the sprocket.
Silent chains often include guide links. Guide links are conventionally flat plates that are positioned on the lateral outside edges of alternate rows of inverted tooth links. The guide links do not form teeth, but generally extend along the row of links and adjacent to the region between teeth formed by the row of links. The guide links on opposite lateral sides of a row are separated by approximately the lateral width of the sprocket teeth that extend between the teeth of the row of links. The guide links thereby act to position the chain laterally on a sprocket (i.e., maintain the chain on the center of the sprocket) but do not engage a sprocket between teeth of the sprocket.
A conventional silent chain drive is comprised of an endless loop silent chain that wraps at least two sprockets. Each sprocket is mounted to a shaft. Rotation of the shaft on which the driving sprocket is mounted transmits power from the driving sprocket through the chain to rotate a driven sprocket and the shaft to which the driven sprocket is mounted. FIG. 1a illustrates such a basic arrangement. A chain 3 forms an endless loop and partially wraps driving sprocket 1 and driven sprocket 2 that are within the loop. A front side 4 of the chain 3 is adjacent to the region within the endless loop formed by the chain 3. Teeth extend inwardly along the front side 4 to engage teeth of both the driving sprocket 1 and the driven sprocket 2. The back side 5 of the chain 3 is the side of the chain opposite the front side 4 and faces outwardly from the chain loop. Both the driving sprocket 1 and the driven sprocket 2 rotate in the same direction, shown counter-clockwise by FIG. 1a. 
Silent chains may also drive sprockets that engage the back side of the chain. Examples of devices that are driven by the back side of a silent chain (back-driven) include water pumps, injector pumps, and countershafts. FIG. 1b shows a sprocket 6 that is back-driven by the chain 3. As in FIG. 1a, a driven sprocket 2 engages the teeth on the front side 4 of the chain 3 and rotates in the same direction as the driving sprocket, counter-clockwise in FIG. 1b. Teeth of the back-driven sprocket 6 engage the back side 5 of the chain 3. The back-driven sprocket 6 rotates in the opposite direction of the driven sprocket 2.
Often, the back side of the links of a chain that engages and drives a sprocket is configured to engage a sprocket as a single tooth between two adjacent sprocket teeth. The teeth of sprockets engaged by the back side of such links are spaced apart by the length of the chain link. Consequently, these sprockets engage a chain link in fewer and farther separated locations than do front driven sprockets. This results in a lower capacity for power transmission by back-driven sprockets. The lower power transmission capacity is acceptable because back-driven sprockets are frequently located in the slack region of the chain and/or are not required to transmit forces that are as large as can be transmitted by the front side of the silent chain. The fewer and more separated contact locations do not control motion of the chain on the sprocket as does contact with a front driven sprocket.
Noise can be generated by a variety of sources in silent chain drives. One significant source of noise is the impact of sprocket teeth on chain teeth at the onset of engagement of the sprocket by the chain teeth. Among the factors that affect the level of the noise created by impact are the impact velocity between the chain and the sprocket and the mass of chain links contacting the sprocket. Noise created by engaging impact in silent chain drives is generally periodic with a frequency generally corresponding to the frequency of the chain teeth engaging sprocket teeth. This frequency is related to the number of teeth on the sprocket and the speed of the sprocket. The impacts can produce sound having objectionable pure sonic tones.
Chordal motion of a chain is another source of noise in power transmission chain drives, including silent chain drives, that is associated with engagement of a chain and sprocket. Chordal motion occurs as a chain link initially engages and begins to move with a sprocket. The sprocket can cause a movement of the span of free chain that is approaching the sprocket along the front to back direction of the chain. This vibratory movement, known as chordal fall, can also produce an objectionable pure sonic tone at the frequency of the chain mesh frequency or a multiple of it.
FIG. 2 illustrates chordal fall. Chains are comprised of a series of pivotally joined rigid links. Consequently, chains are not continuously flexible and do not wrap a sprocket as a circular arc. Rather, a chain wraps around a sprocket in a series of line segments, or chords. As a silent chain engages a sprocket, the individual link teeth contact the surfaces of a sprocket tooth and extend between adjacent teeth of the sprocket. This contact forces the links to articulate around pivot joints with respect to adjacent links. The chain thus bends at the pivot joints between adjacent links as it engages and wraps around a sprocket.
FIG. 2 shows a chord C, which joins the pivot points A and B of a chain link at the position at which the point A engages a sprocket from a free chain run along a direction F. The chord Cxe2x80x2 joins the pivot points at positions Axe2x80x2 and Bxe2x80x2 where the point A has moved with the sprocket half way to the point B location. As shown, by moving from the location A to the location Axe2x80x2 the point A has moved a distance R in the direction T that is perpendicular to the direction F along the free span of the chain. The distance R is the chordal fall of the chain.
Because a chain repeatedly engages teeth, as described above, the change in position of the chain between repeated engagement of the sprocket causes chordal fall. The chordal fall is proportional to the length of C between pivots and inversely proportional to the radius of the circular path traveled by the points A and B around the center of a sprocket. The longer the distance between points at which the chain pivots to engage the sprocket, the chain pitch, the greater chordal fall will be. The smaller the radius of the circular path traveled by the points A and B around the center of a sprocket, the greater the chordal fall will be. For example, a chain with a long pitch engaging a sprocket with relatively small diameter will experience a pronounced amount of chordal fall. In contrast, for an ideal toothless belt engaging a smooth pulley continuously, the length of chord C will be infinitely small, and consequently the belt will experience no chordal fall.
Back-drive chain and sprocket engagement can exacerbate the problem of chordal fall by permitting greater uncontrolled chordal fall movement of a chain on a sprocket than occurs for front drive engagement. A tooth of a front-driven sprocket extends between teeth of a link on the front side of the chain, and two adjacent sprocket teeth engage the link at its ends along the chain direction, one adjacent to each tooth outer flank of the inverted tooth link. A tooth of a back-driven sprocket can only engage the back side of the chain between links along the chain direction. Because the back driven sprocket engages the chain at fewer and more widely separated locations than the front-driven sprocket, the chain frequently moves along the teeth of the back-driven sprocket a greater amount than it does along the teeth of a front-driven sprocket. This movement of the chain along the sprocket teeth in the radial direction of the sprocket is in the approximate direction of chordal fall, and therefore causes the chordal fall motion of the chain to increase due to inability of the chain and sprocket contact to control the chordal fall motion.
One method known in the art to decrease chordal fall motion at back-driven sprockets is to use chains formed by interleaved flat links, that is a flat link is laterally adjacent to ends of two links that are aligned with and adjacent to each other along the chain direction. Sprocket teeth can extend into the back side of the chain at a distance that is only half the length of the link by alternately extending between links of laterally adjacent rows of teeth. The sprocket teeth, however, must be in staggered rows across the lateral direction of the chain. FIG. 3 shows a back drive sprocket with rows of teeth that are staggered, i.e. the teeth rows are not laterally aligned. In this arrangement, sprocket teeth only extend into the back side of the chain between links. However, the distance along the chain direction at which the chain successively engages the back drive sprocket is about half the length of a chain link. Consequently, the back-driven sprocket engages chain at three locations along the chain direction for each row of links.
Staggering the rows of teeth on the sprocket, however, requires an expensive manufacturing process. Because back-driven sprockets typically are not required to transmit high loads, moreover, the additional expense of the staggered row is not required to assure adequate power transmission. Therefore, it is desirable to remove the staggered rows of teeth and replace them with a more cost-effective way of limiting chordal fall motion.
U.S. Pat. No. 1,956,942 discloses a chain that drives a sprocket from both front and back sides, and that engages sprockets on both sides of the chain only between chain links. Links of the chain extend into the sprocket to rest against the flanks of adjacent sprocket teeth facing the link. In one embodiment, a center guide is provided to extend into a recess in the sprocket teeth to prevent the chain from moving laterally on the sprocket. The disclosed chain and sprocket do not provide for guiding the chain around the sprocket other than by engaging the teeth of the sprocket and do not otherwise provide any limitation on the increased chordal fall that results from engaging a sprocket having teeth spaced apart the length of a link.
U.S. Pat. No. 5,967,926 discloses a chain that engages sprockets on both the front side and the back side of the chain and that provides flat surfaces on the back side of the chain to contact a shoe of a tensioner. To avoid the problems that arise from engaging a sprocket only at the ends of the links and still present a back side surface that will smoothly contact a tensioner, two shallow teeth are formed on the back side of the chain links with a flat surface joining the teeth shallow. The sprocket that engages the back side of the chain engages each link at its ends and between the shallow teeth. The sprocket teeth have an arcuate top surface that contacts the flat link surface between the shallow teeth. A guide plate positioned adjacent to these links at the lateral edges of the chain positions a flat surface beyond the teeth to provide smooth contact of the back of the back side of the chain against a tensioner shoe.
Thus, a need remains for a method and apparatus for limiting chordal fall motion of a chain on sprockets having wide tooth spacing such as back-driven sprockets.
In accordance with the present invention, disadvantages of chordal fall motion of chains engaging prior sprockets having wide tooth spacing have been overcome. A sprocket according to the present invention having teeth positioned to engage a sprocket only at positions along the chain that are separated by the length of a link of the chain limits chordal fall motion of the chain.
According to the invention, a sprocket having teeth that extend radially outwardly at the periphery of the sprocket supports a chain engaging the sprocket at a desired radial location along the sprocket teeth. The sprocket teeth are separated from adjacent teeth by a distance that is approximately the length of the links of the chain. The chain engages the sprocket by accepting sprocket teeth between alternating links of the chain. The alternate chain links are positioned for driving contact at opposed faces of a sprocket tooth along the chain direction. A link that is laterally adjacent to a sprocket tooth pivotally engages the two driven links at the opposed sprocket tooth faces. The sprocket and joining link are configured to support the joining link at a desired location radially along the sprocket tooth. By supporting the chain in the radial direction at each sprocket tooth, the sprocket limits motion due to chordal fall in the direction radially along the sprocket teeth.
A preferred embodiment of the invention includes a sprocket having two rows of teeth that are separated from adjacent teeth of the row a distance that is approximately the length of the links of a chain. The rows of teeth are aligned in the lateral direction of the chain. A chain engages the sprocket by positioning driven chain links between adjacent teeth in a sprocket teeth row. Links that are pivotally joined to the driven links are positioned laterally adjacent to the sprocket teeth between sprocket teeth rows. The sprocket further includes an inner guiding shoulder between the rows of sprocket teeth that contacts joining links between sprocket teeth. The shoulder supports the joining link at a desired location along the radial direction of the sprocket teeth and thereby supports driven links that are pivotally joined to the joining link and are driven by the sprocket teeth. The sprocket according to the present invention limits chordal fall motion of the chain on the sprocket by supporting the chain in a radial direction along the sprocket teeth.
A chain and sprocket according to the present invention are particularly well adapted for use in a back-driven application. The objects and advantages of the present invention, as well as details of preferred embodiments thereof, will be more fully understood from the drawings and the following description.